Sleep disorders - other sleep-disordered breathing
Description
- The ICSD-3 categories of sleep-disordered breathing beyond OSA/CSA/obesity hypoventilation (covered in sleep apnoea) - grouped here as sleep-related hypoventilation disorders and sleep-related hypoxaemia
- Unified by a normal or near-normal AHI but abnormal gas exchange during sleep - easily missed if a sleep study is interpreted by AHI alone without reviewing the oximetry/transcutaneous CO2 trace
Epidemiology
- Neuromuscular/chest wall-related hypoventilation - prevalence follows the underlying disease (motor neurone disease, muscular dystrophy, severe kyphoscoliosis, prior polio)
- Opioid-induced central sleep-related hypoventilation - rising with increasing chronic opioid prescribing
- Isolated sleep-related hypoxaemia (without hypoventilation) - seen with severe COPD/ILD where daytime saturation is preserved but nocturnal desaturation is significant
Aetiopathogenesis
Sleep-related hypoventilation
- Neuromuscular/chest wall disease (motor neurone disease, muscular dystrophy, severe kyphoscoliosis, diaphragm dysfunction) - REM sleep selectively impairs accessory/intercostal muscle tone, leaving the diaphragm as the sole pump - decompensates first in REM, explaining early REM-predominant nocturnal hypoventilation before daytime failure develops
- Opioid/sedative-induced - dose-related suppression of central respiratory drive and upper airway tone, can cause an ataxic/irregular breathing pattern distinct from typical CSA
- Idiopathic congenital central hypoventilation - rare, genetic (PHOX2B), presents in infancy
Sleep-related hypoxaemia (without significant hypoventilation)
- Underlying parenchymal/vascular lung disease (severe COPD, ILD, pulmonary hypertension) - ventilation-perfusion mismatch worsens in sleep (reduced tidal volume, reduced FRC, REM-related hypotonia) even without frank hypoventilation or obstructive events
Diagnosis
Key diagnostic principle
- AHI can be normal or near-normal in all of these - the diagnosis is missed if the sleep study report is read for AHI alone rather than the oximetry/CO2 trace
- Sleep-related hypoventilation - defined by a rise in transcutaneous/end-tidal CO2 during sleep (e.g. >=10mmHg rise from awake baseline, or sustained CO2 >50mmHg for a significant proportion of sleep time) on polysomnography with capnography
- Sleep-related hypoxaemia - defined by significant SpO2 <90% for a substantial proportion of sleep time without the apnoea/hypopnoea or hypercapnia pattern that would define OSA/hypoventilation
- Awake ABG/venous bicarbonate - a raised bicarbonate is a practical clue to chronic nocturnal hypoventilation even before formal overnight capnography
- Assess respiratory muscle strength (FVC sitting vs supine - a fall of >25% supine suggests diaphragm weakness) in suspected neuromuscular disease
Management
A. Sleep-related hypoventilation (neuromuscular/chest wall)
- Non-invasive ventilation (bilevel, typically nocturnal initially) - the mainstay once symptomatic nocturnal hypoventilation is confirmed; improves symptoms, gas exchange, and in some conditions (e.g. motor neurone disease) survival
- Multidisciplinary input - respiratory, neurology, physiotherapy (secretion clearance where relevant)
B. Opioid-induced central hypoventilation
- Minimise/reduce opioid dose where clinically feasible, avoid co-prescribing other sedatives (benzodiazepines)
- NIV or CPAP trial in selected persistent cases where the opioid cannot be safely reduced (e.g. palliative/chronic pain settings) - individualised, specialist-guided
C. Sleep-related hypoxaemia
- Nocturnal oxygen supplementation if significant isolated nocturnal desaturation confirmed and daytime hypoxaemia does not already meet long-term oxygen therapy criteria
- Optimise the underlying lung disease (COPD/ILD-directed therapy) - treating the parenchymal disease is often more impactful than oxygen alone
D. General
- Avoid sedatives/alcohol before sleep across all categories - worsens respiratory drive and muscle tone
- Regular follow-up with repeat overnight studies as the underlying disease progresses (particularly progressive neuromuscular disease)
Associations
- Motor neurone disease, muscular dystrophies, severe kyphoscoliosis, prior polio, diaphragm paralysis
- Chronic opioid/benzodiazepine use
- Severe COPD, interstitial lung disease, pulmonary hypertension
- Often coexists with OSA - "overlap" patterns are common and each component may need separate treatment
Natural history & complications
- Untreated chronic nocturnal hypoventilation - progresses to daytime hypercapnic respiratory failure, cor pulmonale, and in neuromuscular disease often heralds a broader decline in respiratory function
- Early NIV initiation in progressive neuromuscular disease (once nocturnal hypoventilation is confirmed) improves quality of life and, in some conditions, survival - a key reason not to wait for daytime respiratory failure to intervene
- Isolated nocturnal hypoxaemia - contributes to pulmonary hypertension and cardiovascular risk over time if untreated, even without daytime hypoxaemia meeting formal long-term oxygen criteria
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